A story I tell you in a bar and a written story may seem like the same thing to you, but one of them is copyrighted and the other isn't. A feature that may seem incidental to you is an essential one to copyright law. To copyright law it matter a great deal exactly how some expression is made, and how it is fixed to some recording medium. In particular, it requires that there exists some particular piece of text that can be called the work. This just doesn't exist for a protocol (there are many original texts that could describe the protocol), but it does exist for a code API.